Emacs Org-Mode is a lot of things; but as the Org-Mode website put it:
Org-mode is for keeping notes, maintaining ToDo lists, doing project planning, and authoring with a fast and effective plain-text system.
Personally, Org-Mode is where I ended up after trying all sorts of desktop and online applications, so I thought I’d share my experience and hope someone else find it useful. The only thing you need to keep in mind while exploring Org-Mode is that, despite how comprehensive and far-reaching it may look, you can start by using only the subset of features (or even the single one) that does what you want. Incremental, on-demand learning is your friend.
These slides were created using org-mode.